The effect of process variables on microstructure in laser-deposited materials

Laser deposition of titanium alloys is under consideration for aerospace applications, which require the consistent control of microstructure and resulting mechanical properties. To date, only limited experimental data exists to link deposition process variables (e.g., laser power and velocity) to resulting microstructure (e.g., grain size and morphology) in laser-deposited materials, and suitable microstructures have typically been obtained only by trial and error. In addition, it is unclear whether knowledge based on small-scale laser deposition processes (e.g., LENS(TM)) can be applied to large-scale (higher power) processes currently under development for commercial applications. Therefore, simulation-based methods are needed to predict the effects of process variables and size-scale on microstructure in laser-deposited titanium and other aerospace materials.; The ability to predict and control microstructure in laser deposition processes requires an understanding of the thermal conditions at the onset of solidification. The focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (laser power and velocity). The approach employs the well-known Rosenthal solution for a moving point heat source traversing an infinite substrate. Cooling rates and thermal gradients at the onset of solidification are numerically extracted from the Rosenthal solution throughout the depth of the melt pool, and dimensionless process maps are presented for both 2-D thin-wall and bulky 3-D geometries. Results for both small-scale (LENS(TM)) and large-scale (higher power) processes are plotted on solidification maps for predicting trends in grain morphology in laser-deposited Ti-6Al-4V. Although the Rosenthal predictions neglect the nonlinear effects of temperature-dependent properties and latent heat of transformation, a comparison with 2-D and 3-D nonlinear FEM results for both small-scale and large-scale processes suggests that they can provide reasonable estimates of trends in solidification microstructure. In particular, both the Rosenthal and FEM results suggest that changes in process variables could potentially result in a grading of the microstructure (both grain size and morphology) throughout the depth of the deposit and that the size-scale of the laser deposition process is important.; In addition, the effects of a uniform distributed heat source on melt pool geometry and microstructure is investigated by superposition of the Rosenthal point source solution. In particular, the effect of beam width on melt pool length, melt pool depth, solidification cooling rates and thermal gradients is investigated. These results are also interpreted in the context of a solidification map to investigate the effect of beam width on trends in grain morphology in laser-deposited Ti-6Al-4V. Finally, transient effects near the free edge are investigated in both 2-D thin-wall and bulky 3-D geometries through thermal finite element analysis. Here the effect of transient melt pool behavior on solidification cooling rates and thermal gradients (and thereby the resulting microstructure) is investigated.